Directory management in Unix can be taken as a representative example of a conventional system having a tree structure. The directory structure of Unix is configured as a structure that is hierarchically arranged by directories from a root node. In the structure, when each user has files such as documents and programs and uses them proactively, the documents and the programs can be shared with other users.
In this case, each of the directories and the files can be provided with attribute settings by users and groups, and can be provided with authority of reading, writing and execution for each of the owner, group and other users (refer to document 1 “Practical UNIX handbook”, Natsume sha, 1994, p 31-42 (ISBN4-8163-1654-X)).